Method and system for measuring noise of a magnetic head

ABSTRACT

A method for measuring noise of a magnetic head includes setting a plurality of threshold values, applying bias current or voltage to a read element of the magnetic head, applying an external transverse magnetic field to the magnetic head, amplifying output signal from the read element to produce an amplified signal, filtering the amplified signal to produce a filtered signal, generating an enable signal for each threshold value in a predetermined time window by a counting control means with input signals which include the filtered signal and the threshold value, measuring the cumulative time duration of each enable signal, making an amplitude-duration distribution according to the cumulative time durations and the threshold values, calculating a plurality of parameters according to the amplitude-duration distribution and analyzing the parameters with a plurality of predetermined criteria to determine the defects of the magnetic head. Accordingly, the invention also discloses a system for measuring noise of a magnetic head.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/923,289filed Sep. 13, 2010, which claims priority to Chinese Application No.201010240216.7 filed Jul. 23, 2010, the entire contents of each of whichis hereby incorporated by reference in this application.

FIELD OF THE INVENTION

The present invention relates to noise-measuring method and system and,particularly, to a method and system for measuring noise of a magnetichead with a magnetoresistive (MR) read element.

BACKGROUND OF THE INVENTION

One known type of information storage device is a disk drive device thatuses magnetic media to store data and a magnetic head that is positionedover the magnetic media to selectively read from or write to themagnetic media.

Ordinarily, a magnetic head consists of a magnetoresistive effect (MR)read element (Reader) and an inductive write element (Writer), whilesome further include a heating unit (Heater). Magnetic head with amagnetoresistive (MR) read element is widely used to hard disk drive(HDD) to increase in capacity and reduce in size. Since then, severaltypes of MR read element have been widely used by disk drivemanufacturers in succession. One is anisotropic magnetoresistive (AMR)element, which makes the angle between the magnetization direction andthe direction of sense current flowing through the MR element changeand, in turn, cause a change the resistance of the MR element and acorresponding change in the sensed current or voltage. Another type isgiant magnetoresistive (GMR) element manifesting the GMR effect. The GMReffect is a phenomenon that the magnetoresistive ratio (MR ratio) willchange under an external magnetic field. The GMR element comprises twoferromagnetic layers and a non-ferromagnetic layer sandwiched betweenthe two ferromagnetic layers. The resistance of the non-ferromagneticlayers varies with the magnetic moments of the ferromagnetic layers, theconduction electrons and the spin-dependent scattering. Still anothertype of MR sensor is tunnel magnetoresistive (TMR) element, which hasbecome the mainstream MR element due to its more remarkable change of MRratio by replacing AMR element and GMR element.

At each test after manufacturing the magnetic head, it is necessary toconfirm that the finished magnetic head with an MR element is a goodarticle and may not produce unacceptable noise. However there are manymechanisms generating noise in an MR read element, some are magnetic innature and some are electrical in nature. The Barkhausen Noise, RandomTelegraph Noise or Popcorn Noise indicates instability in an MR readelement. Instability related noise tends to be a catastrophic problemrather than a contribution to background noise that can limit SNR, thus,it is necessary to handle them with different criteria respectively.

In the art today, different methods and systems are developed to measurenoise in MR heads. The prevalent method is to use a digitizer to convertanalog signal to digital signal, and by using a processor to analyze alloriginal digitized signal as a whole and hence to identify whether themagnetic head produces noise. This conventional method does not handleinstability related noise and background noise respectively, thus it isinadequate to identify true defects accurately. Furthermore, since thenoise of a magnetic head may contain high frequency components, in orderto capture such noise, the digitizer sampling frequency needs to be highenough. That increases testing cost. At the same time, the huge amountof digitized signal generated by the high speed digitizer takes a prettylong time to transfer and process the digitized signal and hence aprocessor with high processing power and enough memory will becomeessential for a faster computation. This also increases testing cost.All these are not in favor of the reduction of production costs.

In response to this, it is desired to provide a method to handleinstability related noise and background noise respectively and acorresponding system to identify true defects fast and accurately togain better cost-effectiveness to overcome the above-mentioneddrawbacks.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a method formeasuring noise of a magnetic head with a magnetoresistive (MR) readelement. The noise-measuring method can handle instability related noiseand background noise respectively to identify true defects accuratelyand can be easily implemented to gain better cost-effectiveness.

Another objective of the present invention is to provide a system formeasuring noise of a magnetic head with a magnetoresistive (MR) readelement. The noise-measuring system can handle instability related noiseand background noise respectively to identify true defects accuratelyand gain better cost-effectiveness by using some lower cost devices.

To achieve above objectives, one aspect of the present invention relatesto a method for measuring noise of a magnetic head. The method includessetting a plurality of threshold values, applying bias current orvoltage to a read element of the magnetic head, applying an externaltime-changing transverse magnetic field to the magnetic head, amplifyingoutput signal from the read element to produce an amplified signal,filtering the amplified signal to produce a filtered signal, generatingan enable signal for each threshold value in a predetermined time windowby a counting control means with input signals which includes thefiltered signal and the threshold value, measuring the cumulative timeduration of each enable signal, making an amplitude-durationdistribution according to the cumulative time durations and thethreshold values, and calculating a plurality of parameters according tothe amplitude-duration distribution and analyzing the parameters with aplurality of predetermined criteria to determine the defects of themagnetic head.

To achieve above objectives, another aspect of the present inventionrelates to a method for measuring noise of a magnetic head. The methodincludes setting a plurality of threshold values, applying bias currentor voltage to a read element of the magnetic head, applying an externalconstant transverse magnetic field to the magnetic head, amplifyingoutput signal from the read element to produce an amplified signal,filtering the amplified signal to produce a filtered signal, generatingan enable signal for each threshold value in a predetermined time windowby a counting control means with input signals which includes thefiltered signal and the threshold value, measuring the cumulative timeduration of each enable signal, making an amplitude-durationdistribution according to the cumulative time durations and thethreshold values, and calculating a plurality of parameters according tothe amplitude-duration distribution and analyzing the parameters with aplurality of predetermined criteria to determine the defects of themagnetic head.

To achieve above objectives, still another aspect of the presentinvention relates to a method for measuring noise of a magnetic head.The method includes obtaining a plurality of first parameters, obtaininga plurality of second parameters and analyzing the relationship betweenthe first parameters and the second parameters with a plurality ofpredetermined criteria to determine the defects of the magnetic head.Wherein obtaining the first parameters by a method including setting aplurality of first threshold values, applying first bias current orvoltage to a read element of the magnetic head, applying an externalconstant transverse magnetic field to the magnetic head, amplifyingfirst output signal from the read element to produce a first amplifiedsignal, filtering the first amplified signal to produce a first filteredsignal, generating a first enable signal for each first threshold valuein a predetermined first time window by a counting control means withfirst input signals which includes the first filtered signal and thefirst threshold value, measuring the first cumulative time duration ofeach first enable signal, making a first amplitude-duration distributionaccording to the first cumulative time durations and the first thresholdvalues, calculating the first parameters according to the firstamplitude-duration distribution and obtaining a plurality of secondparameters by a method including setting a plurality of second thresholdvalues, applying second bias current or voltage to the read element ofthe magnetic head, applying an external time-changing transversemagnetic field to the magnetic head, amplifying second output signalfrom the read element to produce a second amplified signal, filteringthe second amplified signal to produce a second filtered signal,generating a second enable signal for each second threshold value in apredetermined second time window by the counting control means withsecond input signals which includes the second filtered signal and thesecond threshold value, measuring the second cumulative time duration ofeach second enable signal, making a second amplitude-durationdistribution according to the second cumulative time durations and thesecond threshold values, and calculating the second parameters accordingto the second amplitude-duration distribution.

To achieve above objectives, yet another aspect of the present inventionrelates to a system for measuring noise of a magnetic head. the systemincludes a magnetic field generator for applying an external transversemagnetic field to the magnetic head, a reader control unit for applyingbias current or voltage to a read element of the magnetic head, anamplifying unit for amplifying output signal from the read element toproduce an amplified signal, a filter unit for filtering the amplifiedsignal to produce a filtered signal, a threshold control unit forsetting a plurality of threshold values,

a counting control unit for making and outputting a plurality of enablesignals for a plurality of threshold values, a clock unit for makingclock signal, a counter unit for measuring the cumulative time durationof each enable signal, an amplitude-duration distribution processingmodule for generating a amplitude-duration distribution according to thecumulative time durations and the threshold values, and a processor forcalculating a plurality of parameters according to theamplitude-duration distribution and analyzing the parameters with aplurality of predetermined criteria to determine the defects of themagnetic head.

In comparison with the prior art, the present invention measures thetime durations with a plurality of predetermined thresholds to generatean amplitude-duration distribution (histogram) in situ. For eachthreshold, measure the time duration of the part of the tested signalwhich exceeds the predetermined threshold, then a digitizer is notnecessary any more, some other lower cost devices can be used toimplement this measurement, also, there is a significant decrease in theamount of data and data handling time. Based on the amplitude-durationdistribution, the instability related noise and background noise of themagnetic head can be measured respectively to identify true defectsaccurately. Accordingly, the noise-measuring system can use some lowercost devices to handle instability related noise and background noiserespectively to identify true defects accurately in shorter measurementtime to gain better cost-effectiveness.

Other aspects, features, and advantages of this invention will becomeapparent from the following detailed description when taken inconjunction with the accompanying drawings, which are a part of thisdisclosure and which illustrate, by way of example, principles of thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings facilitate an understanding of the variousembodiments of this invention. In such drawings:

FIG. 1 is a functional block diagram illustrating a preferred embodimentof a system of the present invention;

FIG. 2 is a block diagram showing a single-channel counting control unitaccording an embodiment of the present invention;

FIG. 3 is a block diagram showing a multiple-channel counting controlunit according another embodiment of the present invention;

FIG. 4 is a block diagram showing a multiple-channel counting controlunit with an envelope detector according another embodiment of thepresent invention;

FIG. 5 is a block diagram showing a multiple-channel counting controlunit implemented by digital means according another embodiment of thepresent invention;

FIG. 6 is a flow chart schematically illustrating a method according toan embodiment of the present invention;

FIG. 7 illustrates multiple thresholds sweeping with multiple magneticfield cycles according to the method shown in FIG. 6;

FIG. 8 illustrates how to make the amplitude-duration distribution ofthe tested signal according to the method shown in FIG. 6;

FIG. 9 illustrates two amplitude-duration distributions generated by themethod shown in FIG. 6;

FIG. 10 illustrates each amplitude-duration distribution of FIG. 9divided into two different parts of the total population;

FIG. 11 is a flow chart schematically illustrating another methodaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Various preferred embodiments of the invention will now be describedwith reference to the figures, wherein like reference numerals designatesimilar parts throughout the various views.

As indicated above, the present invention provides a method and systemfor measuring noise of a magnetic head. The magnetic head includes amagnetoresistive (MR) read element, a write element and a heater.

FIG. 1 shows a functional block diagram illustrating a preferredembodiment of a system of the present invention. Now referring to FIG.1, the system for measuring noise of the magnetic head 100 including amagnetic field generator, a reader control unit 101, an amplifying unit106, a filter unit 107, a counting control unit 108, a counter unit 109,a threshold control unit 111, a clock unit 110, an amplitude-durationdistribution processing module 113 and a processor 115.

In this embodiment, the magnetic field generator includes anelectromagnet 104 which is provided as a magnetic field source forproducing an external transverse magnetic field to the magnetic head 100and a magnetic field control unit 105 for controlling the electromagnet104. In a preferred embodiment, the magnetic field source is anelectromagnetic-based field source or a permanent magnet-based fieldsource. The reader control unit 101 is provided for applying biascurrent or voltage to the read element of the magnetic head 100.Preferably, the system of the present invention further includes awriter control unit 102 and a heater control unit 103, which are usedfor applying current or voltage to the write element and the heater ofthe magnetic head 100 respectively, thereby the read element being undera stressed condition such that noise is generated with higher chances incertain problematic magnetic head. The reader control unit 101, writercontrol unit 102 and heater control unit 103 are controlled by a headcontrol unit 112. The amplifying unit 106 amplifies the output signalfrom the read element to produce an amplified signal which is sent tothe filter unit 107. The filter unit 107 receives the amplified signalto reject all frequency components including the magnetic filedfrequency outside a band of frequencies to produce a filtered signal.The counting control unit 108 is provided for generating a plurality ofenable signals for a plurality of threshold values which are provided bythe threshold control unit 111. The counter unit 109 is provided formeasuring the cumulative time duration of each enable signal by countingthe cycles of a clock signal provided by the clock unit 110. Accordingto the cumulative time durations and the threshold values, theamplitude-duration distribution processing module 113 generates anamplitude-duration distribution. In this embodiment, theamplitude-duration distribution processing module 113 is implemented bya programmable logic unit 114, thus, the amplitude-duration distributionis made in situ to eliminate the need for further massive data transferand processing. Preferably, the clock unit 110, threshold control unit111, head control unit 112 and the counter unit 109 are the modules ofthe programmable logic unit 114, that is all of them also beingimplemented by the programmable logic unit 114. Preferably, theprogrammable logic unit 114 is a programmable logic device or anembedded microprocessor controlled by the processor 115. The processor115 processes the data received from the programmable logic unit 114 andcontrols the magnetic field control unit 105. In addition, the processor115 calculates a plurality of parameters according to theamplitude-duration distribution, analyzes the parameters with aplurality of predetermined criteria and determines the defects of themagnetic head 100.

Referring to FIG. 2, in an embodiment, the counting control unit 108comprises a D/A converter for generating various analog signalsaccording to defined threshold values and one couple of analogcomparators working together for comparing the amplitude of the filteredsignal with the analog signal successively to produce the enablesignals.

Referring to FIG. 3, in another embodiment, the counting control unit208 is a multiple-channel unit comprising a D/A converter for generatingvarious analog signals according to defined threshold values and severalcouples of analog comparators for comparing the amplitude of thefiltered signal with the analog signals at the same time to reduce theprocessing time. Accordingly, several counter units are provided tocount and store the number of clock cycles generated by the clock unit110 elapsed during the enable signal generated by the different couplesof comparators sustains. As shown in FIG. 4, in another embodiment, thecounting control unit 308 further includes an analog envelope detectorfor obtaining the envelope of the filtered signal.

Referring to FIG. 5, in still another embodiment, the counting controlunit 408 includes an A/D converter for changing the filtered signal tobe digital signal, a digital threshold dispatcher for dispatching thethreshold values and several digital comparators for comparing thedigital signal with the threshold values at the same time to produce theenable signals. Preferably, the counting control unit 408 furthercomprises a digital envelope detector which is set between the A/Dconverter and the digital comparators for obtaining the envelope of thedigital signal from the A/D converter and then sending it to the digitalcomparators. In another embodiment, the digital envelope detector andthe digital comparator are the modules of the programmable logic unit.

As indicated above, in this system of the present invention, combinationof comparator, D/A converter (or A/D converter), counter unit and clockunit are used to implement the time duration measurement, therebyobtaining better performance and higher flexibility. Furthermore, thesystem of the present invention utilizes programmable logic unit with anamplitude-duration distribution processing module to generate ahistogram in situ to save data handling time thereby to reduce the cost.

The present invention further provides a method for measuring the noiseof the head in an external time-changing transverse magnetic field. Inthis embodiment, this method is implemented by the system shown inFIG. 1. Now, referring to FIG. 6 and FIG. 1, the method of the presentinvention includes several steps described as follows.

Step S101 is setting a plurality of threshold values. Step S102 isapplying bias current or voltage to the read element of the magnetichead 100. Concretely, the bias current or voltage is produced by thereader control unit 101 controlled by the head control unit 112. StepS103 is applying an external time-changing transverse magnetic field tothe magnetic head 100. Preferably, the time-changing transverse magneticfield varies in periodic waveform and varies in single cycle or multiplecycles. As shown in FIG. 7, in this embodiment, the time-changingtransverse magnetic field varies in multiple cycles, the thresholdvalues sweep with the time-changing transverse magnetic field. Step S104is amplifying output signal from the read element to produce anamplified signal and filtering the amplified signal to produce afiltered signal.

Step S105 is generating an enable signal for each threshold value in apredetermined time window by a counting control means with input signalswhich includes the filtered signal and the threshold value. Step S106 ismeasuring the cumulative time duration of each enable signal.Preferably, measuring the cumulative time duration of each enable signalis implemented by counting the cycles of a clock signal. In anembodiment, the step S105 is implemented by analog means. The countingcontrol means is an analog means shown in FIG. 2, FIG. 3 or FIG. 4. Inanother embodiment, as shown in FIG. 5, the counting control means isimplemented by digital means. Referring to FIG. 8, at these two steps,each threshold value is compared with the amplitude of the filteredsignal or the amplitude of the envelope of the filtered signal toproduce the enable signal and then the cumulative time duration of theenable signal is measured. In other words, for each threshold value, thecumulative time duration of the tested signal which exceeds thethreshold value is measured by counting the cycle of the clock signal.After that, Step S107 is implemented to make an amplitude-durationdistribution according to the cumulative time durations and thethreshold values.

The last step S108 is calculating a plurality of parameters according tothe amplitude-duration distribution and analyzing the parameters with aplurality of predetermined criteria to determine the defects of themagnetic head 100. Preferably, the parameters comprise a plurality ofcharacteristic values of the total population or a part of the totalpopulation of the amplitude-duration distribution, referring to FIG. 9and FIG. 10. FIG. 9 illustrates two amplitude-duration distributionsgenerated by the programmable logic unit 114 according to the method ofthe present invention. FIG. 10 illustrates each amplitude-durationdistribution of FIG. 9 divided into two different parts of the totalpopulation, e.g., reference population for reference range and extremepopulation for extreme-value range to implement more accurate defectsidentification. Concretely, the characteristic values at least comprisethe peak amplitude, the normalized energy, the average amplitude, theRMS and the SNR. In another embodiment, the parameters comprise therelative relations between the characteristic values of a part of thetotal population of the amplitude-duration distribution and thecharacteristic values of the total population of the amplitude-durationdistribution. In still another embodiment, the parameters comprise therelative relations between the characteristic values of a part of thetotal population of the amplitude-duration distribution and thecharacteristic values of another part of the total population of theamplitude-duration distribution. The characteristic values at leastcomprise the normalized energy and the average amplitude.

As described above, the method of the present invention includesmeasuring the time durations with a plurality of predeterminedthresholds to generate an amplitude-duration distribution (histogram) inreal time. For each threshold, measure the time duration of the part ofthe tested signal which exceeds the predetermined threshold. Thus, basedon the amplitude-duration distribution, the instability related noiseand background noise of the magnetic head can be measured respectivelyto identify true defects accurately.

According to an embodiment of the present invention, another method isprovided for measuring the noise of the head in a constant magneticfield. This method is similar to the method shown in FIG. 6. Now,referring to FIG. 11, the method of the present invention includesseveral steps described detail as follows.

Step S201 is setting a plurality of threshold values. Step S202 isapplying bias current or voltage to the read element of the magnetichead 100. Step S203 is applying an external constant transverse magneticfield to the magnetic head 100. Preferably, the field intensity of theexternal constant transverse magnetic field is zero. Step S204 isamplifying output signal from the read element to produce an amplifiedsignal and filtering the amplified signal to produce a filtered signal.

Step S205 is generating an enable signal for each threshold value in apredetermined time window by a counting control means with input signalswhich includes the filtered signal and the threshold value. Step S206 ismeasuring the cumulative time duration of each enable signal.Preferably, measuring the cumulative time duration of each enable signalis implemented by counting the cycles of a clock signal. After that,Step S207 is implemented to make an amplitude-duration distributionaccording to the cumulative time durations and the threshold values. Inan embodiment, the step S205 is implemented by analog means. Thecounting control means is an analog unit shown in FIG. 2, FIG. 3 or FIG.4. In another embodiment, as shown in FIG. 5, the counting control meansis implemented by digital means.

The last step S208 is calculating a plurality of parameters according tothe amplitude-duration distribution and analyzing the parameters with aplurality of predetermined criteria to determine the defects of themagnetic head 100. Preferably, the parameters comprise a plurality ofcharacteristic values of the total population or a part of the totalpopulation of the amplitude-duration distribution. Concretely, thecharacteristic values at least comprise the peak amplitude, thenormalized energy, the average amplitude, the RMS and the SNR. Inanother embodiment, the parameters comprise the relative relationsbetween the characteristic values of a part of the total population ofthe amplitude-duration distribution and the characteristic values of thetotal population of the amplitude-duration distribution. In stillanother embodiment, the parameters comprise the relative relationsbetween the characteristic values of a part of the total population ofthe amplitude-duration distribution and the characteristic values ofanother part of the total population of the amplitude-durationdistribution. The characteristic values at least comprise the normalizedenergy and the average amplitude.

According to the present invention, still another method is provided formeasuring the noise of the head. This method combines the two methodsmention above. Thus, both of the magnetic noise and electrical noise canbe measured respectively by this method, thereby the true defects of thetested magnetic head can be identified more accurately. The method ofthe present invention includes implementing the steps S101 to S107 toobtaining plurality of first parameters, implementing the steps S201 toS207 to obtaining plurality of second parameters; analyzing therelationship between the first parameters and the second parameters witha plurality of predetermined criteria to determine the defects of themagnetic head 100.

It should be noted that the counter unit 109, the threshold control unit111, the clock unit 110 and the head control unit 112 can be implementedby discrete components. It also should be noted that both of the ordersof the steps S101 to S103 and the steps S201 to S203 can be changedaccording the actual operation.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the invention.

What is claimed is:
 1. A method for measuring noise of a magnetic head,comprising: obtaining a plurality of first parameters by a methodcomprising: setting a plurality of first threshold values; applyingfirst bias current or voltage to a read element of the magnetic head;applying an external constant transverse magnetic field to the magnetichead; amplifying first output signal from the read element to produce afirst amplified signal; filtering the first amplified signal to producea first filtered signal; generating a first enable signal for each firstthreshold value in a predetermined first time window by a countingcontrol means with first input signals which include the first filteredsignal and the first threshold value; measuring the first cumulativetime duration of each first enable signal; making a firstamplitude-duration distribution according to the first cumulative timedurations and the first threshold values; and calculating the firstparameters according to the first amplitude-duration distribution;obtaining a plurality of second parameters by a method comprising:setting a plurality of second threshold values; applying second biascurrent or voltage to the read element of the magnetic head ; applyingan external time-changing transverse magnetic field to the magnetichead; amplifying second output signal from the read element to produce asecond amplified signal; filtering the second amplified signal toproduce a second filtered signal; generating a second enable signal foreach second threshold value in a predetermined second time window by thecounting control means with second input signals which include thesecond filtered signal and the second threshold value; measuring thesecond cumulative time duration of each second enable signal; making asecond amplitude-duration distribution according to the secondcumulative time durations and the second threshold values; andcalculating the second parameters according to the secondamplitude-duration distribution; and analyzing the relationship betweenthe first parameters and the second parameters with a plurality ofpredetermined criteria to determine the defects of the magnetic head. 2.The method as claimed in claim 1, wherein the field intensity of theexternal constant transverse magnetic field is zero.
 3. The method asclaimed in claim 1, wherein the external time-changing transversemagnetic field varies in periodic waveform.
 4. The method as claimed inclaim 3, wherein the external time-changing transverse magnetic fieldvaries in single cycle or multiple cycles.
 5. The method as claimed inclaim 1, wherein the counting control means compares the first thresholdvalue with the amplitude of the first filtered signal or the amplitudeof the envelope of the first filtered signal to generate the firstenable signal and compares the second threshold value with the amplitudeof the second filtered signal or the amplitude of the envelope of thesecond filtered signal to generate the second enable signal.
 6. Themethod as claimed in claim 1, wherein the counting control means isanalog means or digital means.
 7. The method as claimed in claim 1,wherein measuring the first cumulative time duration of each firstenable signal is implemented by counting the cycles of a clock signal.8. The method as claimed in claim 1, wherein measuring the secondcumulative time duration of each second enable signal is implemented bycounting the cycles of a clock signal.
 9. The method as claimed in claim1, wherein the first parameters comprise a plurality of characteristicvalues of the total population or a part of the total population of thefirst amplitude-duration distribution.
 10. The method as claimed inclaim 1, wherein the second parameters comprise a plurality ofcharacteristic values of the total population or a part of the totalpopulation of the second amplitude-duration distribution.
 11. The methodas claimed in claim 9, wherein the characteristic values at leastcomprise the peak amplitude, the normalized energy, the averageamplitude, the RMS and the SNR.